1. Titin is required for lateral stability of the myosin filament lattice in relaxed fibers. The spacing of nearest-neighbor myosin filaments in electronmicrographs of cross sections through the A band was measured in irradiated muscle fibers in the relaxed and rigor states. The irradiation dose was selected to degrade the titin molecules but leave the smaller molecules intact. In relaxed fibers at 20 degrees C, the average filament spacing was the same in irradiated fibers and controls, but the standard deviation of the control fiber spacing distribution was smaller. This result is evidence that the presence of intact titin promotes lateral order in the array of myosin filaments. In rigor fibers the standard deviation of the spacing distribution was the same in irradiated and control fibers, showing that actomyosin crossbridges also stabilize the myofilament lattice. 2. Properties of mutant myosin in muscle fibers from patients with familial hypertrophic cardiomyopathy (FHC). The contractile force, the unloaded contraction velocity, and the ATPase activity were measured in calcium activated, skinned soleus muscle fibers in control fibers and in mutant 741. Force and velocity were down two to three fold in the mutant, but ATPase activity was nearly doubled. It is concluded that in the mechanical and chemical activities of the myosin molecule in the mutant are less tightly coupled than in the normal fiber.